GrapheneOS is pretty neat, https://postmarketos.org/ is also pretty damn polished out of the box these days. I'd argue the "mobile desktop Linux systems" are actually a bit more polished than Graphene, esp. when it comes to default apps (no AOSP abandonware dialer, contacts etc. apps to fight with, it's all just maintained, responsive GNOME/KDE apps)
I tried getting it to run on a Pixel 3a and struggled for hours, eventually gave up (albeit I tried running it with Wayland and Niri which seems less tried-and-true).
I want to believe this, but it's hard for me to take this at face value. It's been about 4 years since I've run pmOS, so my experience IS very out of date, but I also have a hard time believing that the experience has radically changed in the meantime.
The short is that yes the GNOME/KDE apps do often look more impressive, but they suffer the same sort of malaise which seems to have infected Linux desktops sometime since Eternal September, and between the sporadic crashing and "this doesn't feel right", it's really hard for me to accept "It's more polished than AOSP!".
pmOS's installation page opening with a warning:
Which leads to a page that opens with: Does not do a lot to dissuade my skepticism. I know you said the apps specifically, but even there, it's like... I dunno.